Microelectromechanical systems (MEMS) are among the most promising technologies for implementing low-cost, low-power components for radio-frequency RF applications. The micrometric scale of these devices and the possibility of integration can avoid the problem of the large area occupied by the passive components of current RF systems, replacing all of them by a single MEMS chip or integrating them into the processing chip of the system.
Many actuators built with these technologies have already been developed, although only some of them have found a place in the market. A key point of these devices consists on the performance of the actuation method for every specific device. Many actuation principles are used for actuating MEMS: electrothermal, electrostatic, magnetic, piezoelectric, etc. Nowadays, one of the most used actuation principles in micromechanical switches is the electrostatic actuation. In electrostatic actuation a voltage is applied to two layered conductors (electrodes) to induce charge on the conductors, and the force acting between the induced charges is used as an actuating source. Electrostatic actuation has good general properties: large force achieved for small gaps, direct electrical actuation, etc. Nevertheless, a general drawback is usually the need for a large area, which negatively affects many properties of the devices by reducing speed, decreasing reliability and also increasing costs per area. It also has some other disadvantages, like contact related issues or the self-actuation effect and generally medium-high actuation voltages.
Up to now, most of the microdevices which use an electrostatic actuation are based in the use of an electrode at the movable part (like a cantilever beam or a bridge consisting of a beam anchored at both ends) of the device and a fixed electrode at the substrate.
Current microswitches usually suffer the problem of a deficient contact surface, which in general causes effects like higher switch on-resistance and severe contact degradation. One of the reasons is the curled surface of the movable part when contact happens. This problem is usually solved in part by applying a higher voltage than it is actually needed; but this can have negative effects on the switch performance. Some other times this is solved by using a bulky or large central region of the movable part of the switch.
Another common problem is the self-actuation effect of the switch due to the signals present in the line(s) to be switched. If the signal is large enough, the switch may undesirably close, causing malfunctioning.
FIG. 4a-4e of U.S. Pat. No. 5,619,061 show a representative case of state of the art microswitches. The working principle is also representative of the state of the art, where the actuating electrodes 405 and 406 exert an electrostatic force over the movable part until a contact is achieved between contact lines 402 and 403 and the movable part 414 and 412. Topologically, the actuating electrodes 405 and 406 and the contact lines 402 and 403 are located at the same height level, and under the movable part 412.
Also, in the way of an example, U.S. Pat. No. 6,784,769-B relates to a microswitch that includes two distributed constant lines disposed close to each other, and a movable element arranged above them, and a driving means (4) for displacing the movable element by an electrostatic force to bring the movable element into contact with the distributed constant lines. The movable element has two projection formed by notching an overlap portion of the movable element which is located on at least one distributed constant line. The projections oppose a corresponding distributed constant line.
U.S. Pat. No. 5,801,472-A describes a micro device with integrated electrostatic actuator, with a fixed portion and a movable portion which are opposite. The relative amount of movement is controlled by controlling electrostatic force operating between both; the movable portion is moved by the Integrated electrostatic actuator and a portion connected to the movable portion which can be operated mechanically. The probe of a scanning probe microscope Is provided to the movable portion of the above actuator. The above transducer is provided with the structure in which a large number of such actuators are arranged two- or one-dimensionally.
Document US-2003/015936-A1 discloses an electrostatic actuator. A multi-layered auxiliary electrode is further arranged between a main electrode and an actuating body, and positive charge or negative charge is applied to the main electrode, respective auxiliary electrodes, and the actuating body such that electrostatic attractive force is generated between the auxiliary electrodes adjacent to the main electrode, between adjacent auxiliary electrodes, and between auxiliary electrodes adjacent to the actuating body.